1. Field of the Invention
The present invention relates to a method and system for measuring the precision in the indexing angle of a table of a machine tool on which a workpiece or tool is placed. More particularly, this invention is concerned with a precision indexing angle measuring method and system for machine tools enabling measurement of the precision in an indexing angle within a range of 360.degree. or more by utilizing a relative angle measurement apparatus capable of measuring a relative angle with high precision within a given range of measurable angles.
2. Description of the Related Art
A machine tool such as a machining center has a table on which a workpiece is placed. The table can be rotated. The machining center is designed to be large in scale and to operate with high precision. It is required to improve the precision in an indexing angle in the course of setting the table at a given angle. Some large-scale machining centers have an indexing angle resolution, that is, the ability to index an angle, in the order of several seconds. It is necessary to inspect the precision in indexing the angle of an actually-manufactured table, and it is therefore required to measure the angle of rotation of the table for an increment of angle equal to or shorter than the indexing angle resolution of the machine.
For measuring the precision in indexing the angle of a table, a method adopted in the past is such that, for example, a mirror is immobilized on the table; laser light is irradiated externally to the table; and the position of laser light reflected from the mirror is detected. For ready detection of the position of laser light, the laser light is scanned, and the position of the laser light is detected according to specific detection timing. By the way, the precision in indexing the angle of the table must be measured with a fine intervals over one turn, that is, 360.degree. C. However, in the known method of detecting the position of laser light reflected from the mirror on the table, the interval is determined by a number of facets of the polygon mirror. For example, the interval is 60.degree. C. when the number of the facets is six. The number of the facets must be increased in order to reduce the interval. However, this increases a cost of the polygon mirror. Namely, the known method using the polygon mirror has a problem that measurement cannot be carried out with a fine interval.